Recovery of gas oil from a fluid coker



Aug. 9, 1960 w. w. BRAY, JR., ETAL 2,948,670

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inventors Murdo M. MucFurquhor y 25. CW Ai'iorney 294 x701 RECOVERY onGAS on. FROM A rum) CoKER Claims. cl. 208-100) The present inventiondeals with a method and ,apparatus for improving the fractionation ofdesired materials from hydrocarbon vapors. More particularly, it isconcerned with improving the recovery of gas oil fractions' whilesimultaneously quenching heavy normally liquid materials.-

In its broadest aspect, the present invention is applicable to anysystem employing a recovery step to fractionate hydrocarbons so as torecover particular constituents. Thus, it may be applied to conventionalfractionating units, combination fractionators, vacuum pipestills,superposed scrubbers operating in conjunction with inert or catalyticreaction units, and the like. It is specifically advantageous inprocesses for the conversion of heavy hydrocarbon oils to lightermaterials and substantial portions of .gas oils, as for example, in thefluid bed coking process.

While the invention is not restricted to cfluid coking of heavy oils, abrief description of this operation will be given for sake ofemphasizing the considerable commercial advantages afforded by thepresent invention.

In this process, a heavy oil such as raw crude, vacuum bottoms or thelike is introduced into a turbulent mass of inert solids, normallymaintained at a temperature of 900-1200 F. While coke granules arepreferred, other inert solids such as glass, ceramic beads, sand, etc.may be alternatively employed as the bed solids. Upon contact with thehot solids, the oil is converted to vaporous materials, and carbonaceousresidue which deposits on the bed particles. Heat for the processisnormally provided by passing contact solids to a combustion zone whereinoxidation of carbonaceous deposits thereon serves to heat the particles.Recirculation to the reaction bed provides necessary thermal energy forthe conversion step.

The hot coker vapors, generally after a solids separation step, are thenintroduced into a fractionation tower, normally in the form of asuperposed scrubbing 'unit in contiguous relation to the coking reactoritself. Conventionally, a shed section is provided above the point ofproduct vapor introduction to the fractionator in order to encourage therectification of upflowing vaporous reaction products and downco'ming'condensate, the con- .densate resulting from the natural tendency of thevapors to cool, and the recycling of externally cooled reflux materials.Gas oil product is withdrawn as a sidestream while naphtha, gas andlight vapors are taken overhead. Heavy liquid ends are generallydischarged fronrthe bottom of the fractionator and at least a portioncustomarily recycled to the coking bed.

In the past, considerable difiiculty has been encountered in obtainingdesired yields" of gas oil. An appreciable portion of the gas oilproduct is never fully recovered from the heavy liquid ends due tolimitations in the frac-.

tionation system. It has been estimated that 30% of theoretical gas oilyield may be lost in this manner. Unrecovered-coker gas oil represents-aserious economic loss since it is a relatively high valued product whichmay 2 be used as catalytic cracking or thermal crackingsfeed.

The gas oil yield is dependent in large measure upon thevaportemperature and gas oil partial pressure below and at the point ofwithdrawal from the system. Maximum allowable vapor temperatures below"the gas oil draw-0E in a coker scrubber are limited by the maximumallowable liquid temperatures that can be tolerated before cokingoccurs. In an attempt to maintain tolerable temperature levels,heretofore, cool liquid 'bottoms have been returned to the scrubber.However, this method has several disadvantages. It tends to cause poormass transfer from vapor to liquid in the shed section since the vaporsare contacted with a recycled liquid rather than a fresh contactingmedium. Concentration driving forces are thus relatively small. A highpreponderance of heavy liquid ends further tendsto promote cokeformations. Additionally, extraneous equipment in the way of pumps,condensers and the like are necessary to recycle this liquid quenchingagent.

The present invention sets forth means whereby not only is thetemperature level of downcoming liquid ends controlled, but substantialadditional benefits secured. In accordance with the present invention, asection of the fractionation vessel hereafter referred to as a mix ing'zone, is provided between the lowermost portion of the fractionator andthe inlets of the hot, product vapors. A relatively cool gas isintroduced into'this mixing section, and serves to strip downcoming,counterflowing liquids of gas oil while simultaneously acting as aquenching agent. The gasundergoes heating during its upward passage andenters the area of product vapor introduction substantially at the sametemperature as the product materials. The thus preheated gas jthenserves to reduce the partial pressure'of the gas oil, thereby iniprovingthe effectiveness of gas oil'recovery. Gas oil out point is increasedand higher overall yields obtained. r

It is readily apparent that the present invention has broadpracticality, and may be employed in any hydrocarbon fractionationsystem where quenching, stripping and partial pressure reduction aredesired.

The various aspects, of the present invention will become more apparentin light of the following drawings, discussion, and accompanyingexamples.

'Figure 1 illustrates its application in a fluid cokingscrubbing unit; r

Figure .2 depicts a combination fractionator operating in accordancewith the present invention.

Turning to Figure 1, there is shown one embodiment of the presentinvention, as applied .to' the heretofore described fluid bed coking ofheavy hydrocarbon oils". The system essentially comprises areactorsve'ssel' 1, only .the'upper portion of which is indicated,andsc'rubber-fractionator unit 2. Hot coker vapors at a temperature ofabout 975 F;, pass upwardly fromthe reaction bed, through a dilute phasezone and then generallythrough a solids separation unit, not shown.Thereafter they are introduced into scrubber section 2 byone ormorevapor conduits 3-. It'is generally preferred to introducethe vapors intothe scrubber at an inclined angle so as to prevent backfiow of liquidinto the passageway. The hot vapors comprise cracked hydrocarbonproducts, relatively heavy volatilized fractions and fiuidizing gases,such as steam,nitrogen or the like, employed to maintain" the reactionbed in turbulent condition. r

As in conventional practice, a shed section, containing disc anddoughnut structures 4 or similar vapor;

3 propriately introduced cool recycle materials, as is known in theart.' Volatile fractions such as naphthas and lighter constituents arewithdrawn overhead by passageway 9 and subjected to condensation,compressing, etc. to recover desired products.

In accordance with the present invention, a relatively cool gas isintroduced into a mixing zone 13 located in the lower portion of thescrubber unit, below the hot vapor inlets. The cool gas is preferably atan initial temperature in the range of 60 to 300 F., although inpractice it may be at any temperature lower than downflowing hydrocarbonliquid. In order to secure the beneficial partial pressure reductionfeatures of the present invention, the cool gas should have anatmospheric boiling point of less than 300 F. preferably below 200 F.Put in other words, it should be sufficiently lower boiling than gas oilso as to appreciably depress its partial pressure. Thus materials suitedfor introduction are, among others, steam, nitrogen and C and lighterhydrocarbon gases. Light hydrocarbons may be advantageously supplied byrecycling a portion of the gases withdrawn overhead from the scrubberafter subjecting them to sufiicient cooling.

As shown, the cool gas is dispersed over the cross section of thescrubber by means of nozzle finger 12 or other appropriate gas injectionmeans located on the terminal portion of cool gas inlet 11. In additionto, or instead of employing a nozzle structure, mixing section 13 may beadapted to contain bubble plates, disc and doughnuts, etc. for promotingthe intermingling of upfiowing cool gas and downcoming liquid fractions.

Thus, the cool gas serves to quench the downcoming hydrocarbon liquid toa temperature of about 700 F., the liquid settling and accumulating toform bottoms holdup 10. Additionally, the gas acts as a stripping agent,recovering gas oil and similar valuable materials from the heavy liquidends. During the course of its passage upwardly through the mixingsection, the cool gas is preheated and is substantially at thetemperature of the hot coker vapo'rs, i.e. 700 to 800 F., by the time itreached the point of introduction of the product vapors. Thus, it doesnot appreciably cool incoming pro'duct vapors, nor cause carbondepositing condensation. Thereafter, the cool gas of the presentinvention serves as a gas oil partial pressure depressant. As will beappreciated by those in the art, decreased gas oil partial pressureresults in an increased gas oil cut point with consequent increasedyields of desired gas oil products. Generally, the gas oil partialpressure may be reduced by as much as .075 atm., cut points beingthereby increased from approximately 1000" F. to 1050 F.

After passing through additional rectification configurations, gas oil,with a cut point in the neighborhood of 1050 F., is withdrawn fromdraw-01f pan 6 by line 7. It may then be passed to storage, a catalyticcracking unit, or other processing. Customarily, a portion is recycledby line 8 back to this scrubbing unitas a rectification aid.

Bottom liquid is withdrawn by line 14. While the use of liquid quenchingmay be entirely eliminated in accordance with the present invention, thedrawing depicts a highly flexible arrangement wherein a portion of theheavy liquid is passed by conduit 15 to cooling in condenser 16,whencefrom it is recycled through passageway 17 back into the shedsection of the scrubber. Additional reflux and cooling may thus besupplied.

Figure 2 illustrates utilization of the present invention in acombination fractionator 100. As is conventional in such a unit, hotvaporous materials, such as the superheated vapors from a reformer,catalytic or inert reaction step, visbreaker, etc. are passed by line101 into the shed section 102 of the fractionator, wherein they contactdowncoming liquid oil supplied by the introduction of reduced crudethrough line 103. The vapors serve to heat the reduced crude, thusvaporizing .4 out desirable hydrocarbon fractions in gas oil or thelike.

According to this invention, a cool gas of the type previously describedis injected through line 104 and multiple dispersion means 105 into thefractionator below the hot vapor inlet. Downcoming liquid is thussimultaneously quenched and stripped as it descends to holdup section106. Liquid in bottom reservoir 107 formed therein may be intermittentlyor continuously withdrawn through conduit 109 by suitable manipulationof valve 108.

The cool gas, continuously heated in its upward passage, contacts thefeed vapors in section 102, thereafter serving to depress hydrocarbonpartial pressure. Total gas effluent may be removed through passageway110.

As previously noted, various structural aids for promoting rectificationmay be employed in the mixing zone between cold and hot vapor inlets.

The following table sets forth a compilation of pertinent conditionsamenable for operating in accordance Thus, it is seen that by operatinga hydrocarbon recovery system in accordance with this invention,downcoming liquid fractions are both quenched and stripped, andhydrocarbon partial pressure reduced thereby permitting increased cutpoints or yields. Particularly, gas oilrecovery is substantiallyenhanced with a minimum of extraneous equipment or cumbersomeprocedures.

Having described the present invention, what is claimed is as follows:

1. A process for improving recovery of gas oil from hot vaporous crackedproducts comprising the efiiuent from a fluid coking reaction zone,which comprises passing such hot vaporous cracked products directly fromsaid coking zone into a region above the bottom of a fractionating zoneand above a mixing section therein for upward flow through saidfractionating zone for contact with downflowing liquid hydrocarbonstherein including condensate oil and for rectification of desiredfractions, collecting a residual bottoms fraction in the bottom of saidfractionating zone, removing uncondensed vapors and gases overhead fromsaid fractionating zone, cooling the introduced vaporous crackedproducts with said downflowing liquid hydrocarbons and enhancingrecovery of a gas oil hydrocarbon fraction by introducing above thelevel of said collected bottoms fraction a light hydrocarbon gas havingan atmospheric boiling point of less than 200 F. and at a temperaturebelow about 300 B. into said mixing section arranged in the bottomportion of said fractionating zone, contacting said relatively coolhydrocarbon gas with said downflowing liquid hydrocarbons in said mixingsection so as to cool said downflowing liquid hydrocarbons and stripvolatile material therefrom while heating said hydrocarbon gas tosubstantially the temperature of said vaporous cracked products by thetime said hydrocarbon gas reaches the region of entrance of saidvaporous cracked products to said fractionating zone, and contactingsaid thus heated hydrocarbon gas with said hot vaporous cracked productsand said downflowing liquid hydrocarbons as said heated. hydrocarbon gaspasses upwardly through said fractionating zone to depress thehydrocarbon partial pressure of vaporous gas oil hydrocarbons passing upthrough said firactionating zone without condensing gas oil hydrocarbonsto increase the recovery of a liquid gas oil fraction further up saidfractionating zone, said hydrocarbon gas being selected from the groupconsisting of C and lighter hydrocarbon gases, and removing said liquidgas oil fraction from the upper portion of said fractionating zone.

2. A process for increasing the recovery of gas oil from hot vaporouscracked products comprising the effluent from a fluid coking reactionzone which comprises passing such hot vaporous cracked products directlyfrom said coking zone into a region above the bottom of a fractionatingzone and above a mixing section therein for upward flow through saidfractionating zone for contact with downflowing liquid hydrocarbonstherein including condensate oil and for rectification of desiredfi-actions, collecting a residual bottoms fraction in the bottom of saidfractionating zone, removing uncondensed vapors and gases overhead fromsaid fractionating zone, cooling the introduced vaporous crackedproducts with downflowing liquid hydrocarbons and enhancing recovcry ofa gas oil hydrocarbon fraction by introducing above the level of saidcollected bottoms fraction light hydrocarbon gas having an atmosphericboiling point of less than 200 F. and having been cooled to atemperature in the range of 60 F. to 300 F. into said mixing sectionarranged in the bottom portion of said fractionating zone, contactingsaid relatively cool hydrocarbon gas with said downfiowing liquidhydrocarbons in said mixing section so as to cool said downflowingliquid,

hydrocarbons and strip volatile material therefrom while heating saidhydrocarbon gas to substantially the temperature of said vaporouscracked products by the time said hydrocarbon gas reaches the region ofentrance of said vaporous cracked products to said fractionating zone,andcontacting said thus heated upflowing hydrocarbon gas with said hotvaporous cracked products to depress the hydrocarbon partial pressure ofvaporous gas oil hydrocarbons passing up through said fractionating zone6 without appreciably cooling said vaporous cracked vapors to increasethe recovery of a liquid gas oil fraction further up said fractionatingzone, said hydrocarbon gas having been recovered from said uncondensedvapors and gases passed overhead from said firactionating zone, andrecovering an increased yield of said liquid gas oil fraction from anupper portion of said =fractionating zone.

3. A process according to claim 2 wherein hydrocarbon oil feed isintroduced into the upper portion of said fractionating zone fordownward flow therethrough in countercurrent to upflowing vapors andgases to remove volatile components from said oil feed.

4. A process according to claim l wherein said hot vaporous crackedproducts introduced into said fractionating zone are at a temperaturebetween about 700 F. and 800 F.

5. A process according to claim 2 wherein said but vaporous crackedproducts introduced into said fractionating zone are at a temperaturebetween about 700 F. and 800 F.

References Cited in the file of this patent UNITED STATES PATENTS BoreyFeb. 10, 1959

1. A PROCESS FOR IMPROVING RECOVERY OF GAS OIL FROM HOT VAPOROUS CRACKEDPRODUCTS COMPRISING THE EFFLUENT FROM A FLUID COKING REACTION ZONE,WHICH COMPRISES PASSING SUCH HOT VAPOROUS CRACKED PRODUCTS DIRECTLY FROMSAID COKING ZONE INTO A REGION ABOVE THE BOTTOM OF A FRACTIONATING ZONEAND ABOVE A MIXING SECTION THEREIN FOR UPWARD FLOW THROUGH SAIDFRACTIONATING ZONE FOR CONTACT WITH DOWNFLOWING LIQUID HYDROCARBONSTHEREIN IN CLUDING CONDENSATE OIL AND FOR RECTIFICATION OF DESIREDFRACTIONS, COLLECTING A RESIDUAL BOTTOMS FRACTION IN THE BOTTOM OF SAIDFRACTIONATING ZONE, REMOVING UNCONDENSED VAPORS AND GASES OVERHEAD FROMSAID FRACTIONATING ZONE, COOLING THE INTRODUCED VAPOROUS CRACKEDPRODUCTS WITH SAID DOWNFLOWING LIQUID HYDROCARBONS AND ENHANCINGRECOVERY OF A GAS OIL HYDROCARBON FRACTION BY INTRODUCING ABOVE THELEVEL OF SAID COLLECTED BOTTOMS FRACTION A LIGHT HYDROCARBON GAS HAVINGAN ATMOSPHERIC BOILING POING OF LESS THAN 200*F. AND AT A TEMPERATUREBELOW ABOUT 300* F. INTO SAID MIXING SECTION ARRANGED IN THE BOTTOMPORTION OF SAID FRACTIONATING ZONE, CONTACTING SAID RELATIVELY COOLHYDROCARBON GAS WITH SAID DOWNFLOWING LIQUID HYDROCARBONS IN SAID MIXINGSECTION SO AS TO COOL SAID DOWNFLOWING LIQUID HYDROCARBONS AND STRIPVOLATILE MATERIAL THEREFROM WHILE HEATING SAID HYDROCARBON GAS TOSUBSTANTIALLY THE TEMPERATURE OF SAID VAPOROUS CRACKED PRODUCTS BY THETIME SAID HYDROCARBON GAS REACHES THE REGION OF ENTRANCE OF SAIDVAPOROUS CRACKED PRODUCTS TO SAID FRACTIONATING ZONE, AND CONTACTINGSAID THUS HEATED HYDROCARBON GAS WITH SAID HOT VAPOROUS CRACKED PRODUCTSAND SAID DOWNFLOWING LIQUID HYDROCARBONS AS SAID HEATED HYDROCARBON GASPASSES UPWARDLY THROUGH SAID FRACTIONATING ZONE TO DEPRESS THEHYDROCARBONS PARTIAL PRESSURE OF VAPOROUS GAS OIL HYDROCARBONS PASSINGUP THROUGH SAID FRACTIONATING ZONE WITHOUT CONDENSING GAS OILHYDROCARBONS TO INCREASE THE RECOVERY OF A LIQUID GAS OIL FRACTIONFURTHER UP SAID FRACTIONATING ZONE, SAID HYDROCARBON GAS BEING SELECTEDFROM THE GROUP CONSISTING OF C4 AND LIGHTER HYDROCARBON GASES, ANDREMOVING SAID LIQUID GAS OIL FRACTION FROM THE UPPER PORTION OF SAIDFRACTIONATING ZONE.